I feel like it’s a HUGE (silly) assumption you’d ask generically “did you send this email” instead of something more specific like “do you REALLY want me to transfer you money like this?” to which the manager would obviously be confused and the attack would likely be killed in that conversation.
This is an interesting attack vector but I am questioning how likely it is to succeed. The article paints a very specific and narrow window of events for this attack to really work. I don’t buy it, personally.
EDIT: I know phishing happens and works. I am not saying it doesn't. I just mean the people that fall for phishing don't need this sophisticated of an attack to fall for. In fact, the attacker probably narrows the chance of success by putting this much extra (very specific) effort into the attack. They are likely to just succeed with their typical phishing email.